Middle-aged and elderly individuals (aged 45 years or above) currently account for 13% of the global population and are predicted to account for 21% by 2050 (Harper, 2014; Singh et al., 2015; United Nations, 2011). Such individuals commonly experience deterioration of the range of motion (ROM) of joints (Nonaka et al., 2002; Shields et al., 2010). While severe deterioration of joint ROM generally occurs after the age of 71 years, the onset and rate of progression of such degradation in the joints of the upper and lower body vary in each individual (Stathokostas et al., 2013). Reduced ROM is associated with increased risk of falling among middle-aged and elderly individuals (American College of Sports Medicine, 2013), and falls are recognized as a major risk factor for accidental death and trauma in the elderly (World Health Organization, 2007). Indeed, the World Health Organization reported falling as the second cause of accidental injury and revealed that, every year, one of every three elderly individuals would experience a fall (World Health Organization, 2007; Zhao and Chung, 2016).
Stretching can maintain joint flexibility and ROM, thereby effectively decreasing the risk of injury (Behm et al., 2016) and increasing the quality of body movement. Static stretching (SS) can improve joint ROM (Behm et al., 2011; 2016; Bouvier et al., 2017; Kay et al., 2012; Reid et al., 2018) and prevent damage to the muscle and tendons, thus serving as the safest form of stretching (Beaulieu, 1981; Weerapong et al., 2004). Dynamic stretching (DS) can also improve joint ROM, achieve the warm-up effect relatively quickly, promote flexibility, and decrease passive muscle tension (Chen, 2006; Stanziano et al., 2009; Weerapong et al., 2004; Yamaguchi and Ishii, 2005). Meanwhile, previous studies indicated that strength training is associated with increased adaptation response of neuromuscular and connective tissue, as well as with improved flexibility of tendons and ligaments (Fowles et al., 2000; Kubo et al., 2002; Simao et al., 2011). Several studies have confirmed that strength training with appropriate loading improves joint ROM (Leite et al., 2017; Simao et al., 2011). Morton et al. (2011) have suggested that 5-week resistance training regimens involving appropriate and full joint ROM improve flexibility. Leite et al. (2017) have observed that 72 sessions of resistance training confer increased joint flexibility. Swank et al. determined that the Body Recall program, which consists of strength training, posture exercise, and breathing exercise three times per week for 10 weeks, improved hip flexion ROM when the lower limbs were loaded with 0.91 kg (Swank et al., 2003). However, Raab et al. (1988) observed no beneficial effect on hip flexion ROM for training with a 2.15-kg load, possibly because this load may have been excessively heavy for elderly individuals with relatively low muscle strength.
Morton et al. (2011) agreed that, if stretching is conducted within an appropriate and full joint ROM, it may positively influence the efficiency of joint motion. However, previous studies did not document the motion angles of the hip joint during DS and did not evaluate whether the participants achieved adequate and full joint ROM during DS with various loads. Therefore, it remains unknown whether the effectiveness of stretching interventions for ROM improvement is related to the loads used. We hypothesized that, compared to SS, DS would demonstrate more immediate response and sustained effects for improving hip flexion and extension in the elderly, and that the effect of DS would differ with the load used. The present study evaluated the immediate response and sustained effect of a single session of SS and DS with various loads, analyzed the kinematics parameters of stretching motion under various loads, and determined the optimal stretching strategy for increasing hip joint ROM in elderly individuals through group exercise courses.
Sixteen participants (age, 63.2 [+ or -] 7.13 years; body mass index, 21.7 [+ or -] 6.81 kg/[m.sup.2]) were recruited in this study. Each participant performed a single session of each type of stretching exercise, in random order. The inclusion criteria were as follows: (a) absence of conditions possibly affecting hip joint flexion or extension, including problems with the upper limb, lower limb, or back skeletal muscles; (b) independent ambulation; (c) independent, community-living; (d) absence of severe cardiovascular disease or central nervous system disease. Prior to initiating the study, all prospective participants received detailed instructions and were informed of the study procedure, as well as of the benefits and risks of the investigation; those who chose to participate signed an institutionally approved informed consent document to participate in the study. This study was approved by the research ethics committee of the local university (NTU-REC No.: 201305HS008).
First, the reliability of hip joint ROM measurements was assessed using intraclass correlation coefficients. Subsequently, precision was assessed in terms of coefficient of variation. The ROMs of flexion and extension in the right hip joint were measured in eleven participants for four different stretching modes. The intraclass correlation coefficients for hip flexion and hip extension ROM measurements were 0.97 and 0.73, respectively (n = 44). Regarding measurement precision, the coefficients of variation for hip flexion and hip extension ROM were 3.07% and 7.65%, respectively (n = 44).
Each participant performed four stretching exercises, namely SS, dynamic stretching with no-load (DSNL), DS with a light load (DSLL, 0.25 kg), and DS with a heavy load (DSHL, 0.5 kg), in random order. DSLL and DSHL were conducted with light packs (0.25 kg) and heavy packs (0.5 kg), respectively, fixed at the ankle. DSNL was conducted without any pack at the ankle. Measurements were conducted at the same time each day. The time interval between tests was 48 h, and all four exercises were completed within 1 month. In this study, SS was designed to stretch the hamstrings by adopting a forward flexion position while sitting in a chair, and to stretch the iliopsoas by adopting a forward lunge position. Each SS exercise set included six 30-s long repetitions of stretching exercise, with 30 s of rest between repetitions. The three repetitions for iliopsoas stretching were performed after the three repetitions for hamstring stretching. DS was designed to stretch the hip flexors and extensors by adopting a neutral standing posture centered over the left foot while holding onto the...